Tantalum powder and niobium powder have been widely used as materials for an anode electrode for solid electrolytic capacitors. To manufacture tantalum powder or niobium powder for use as an anode electrode in a solid electrolytic capacitor, for example, tantalum fine powder or niobium fine powder can be obtained first by a sodium reduction method of tantalum salt or niobium salt in diluent salt or a hydrogen reduction method of tantalum chloride or niobium chloride. Next, using the tantalum fine powder or the niobium fine powder as raw powder, granulated powder can be formed by a pan-type granulator. After thermal aggregation of the granulated powder, the obtained agglomerate powder can be crushed by a crusher, such as a chopper mill. Then, the obtained crushed powder can be sifted, and powder of a given diameter range is recovered to form a product (for example, see Japanese Unexamined Patent Application Publication H4-362101). In addition, for purposes of increasing the yield of powder having a given diameter range, powder outside of the given diameter range may be recycled. In particular, powder larger than the given diameter range may be mixed with an agglomerate powder to be crushed again, while powder smaller than the given diameter range may be mixed with tantalum raw powder to be granulated again. However, since the crusher used by conventional manufacturing methods has a strong impact on crushing the powder, the particle size distribution of the obtained powder broadens. Thus, it has been difficult to obtain a powder having a particular diameter range at high yield.
In conventional manufacturing methods, repeating granulation or crushing several times to increase the yield of powder having a particular diameter range tends to decrease the unevenness of the powder surface, resulting in a shape inconvenient for thermal aggregation. Therefore, the number of times of re-granulation and re-crushing has been limited, and so the hard-to-thermally agglomerate tantalum powder has been returned to tantalum salt, for eventual use as a material for tantalum raw powder. Accordingly, in conventional manufacturing methods, to increase the yield of powder of a given diameter range, there are limitations on improving the yield, in addition to the need for a lot of work and time.
Tantalum powder and niobium powder obtained by conventional manufacturing methods can have many surface concaves and convexes and the surface can be markedly uneven. Since powder of such shape has a large contact area to contact with other powder, the sinterability required for the powder used for an anode electrode can be ensured. However, there is a problem in that the fluidity is low due to the large flow resistance.
On the other hand, using rough spherical powder is one way to increase the fluidity. However, in that case, the contact area of particles is reduced, resulting in a decrease in the sinterability. In other words, it is conventionally difficult to obtain tantalum powder and niobium powder that achieve both sinterability and fluidity.
With respect to tantalum powder or niobium powder for use as an anode electrode in a solid electrolytic capacitor, for example, it is useful that the powder have a small diameter so as to be easily filled in a small die for making of the electrode, as well as a narrow particle distribution range. Therefore, for example, to manufacture a tantalum powder for an anode electrode, the above manufacturing method has been employed.
In a solid electrolytic capacitor, a tantalum or niobium porous sintered body can be used for an anode, an oxide film formed from the anode surface can be used as a dielectric layer, and a conductive polymer or other material can be used as a cathode. For example, solid electrolytic capacitors can be manufactured by sintering a tantalum or niobium powder to obtain a porous sintered body, oxidizing the surface of the porous sintered body by electrolytic oxidation and the like to form a dielectric layer, and impregnating a solution containing a conductive polymer (hereinafter, referred to as a conductive polymer-containing solution) into the dielectric layer to form a cathode.
Recently, further demand has been made for a high-volume capacitor. In order to increase the surface area of the dielectric layer, there has been a trend towards using smaller diameter tantalum powder or niobium powder for molding a porous sintered body, resulting in a porous sintered body with smaller holes or pores. However, the viscosity of the conductive polymer-containing solution can be high and, therefore, if the holes or pores of the porous sintered body are small, it becomes hard for the conductive polymer-containing solution to impregnate into the holes or pores, which inhibits the sufficient molding of the cathode into the holes or pores. As a result, there has been a problem in that the equivalent series resistance (ESR) of the solid electrolytic capacitor increases.
Japanese Unexamined Patent Application Publication 2001 345238 proposes a method of manufacturing a porous sintered body by adding an acid dissolving or a heat sublimation hole molding material (hole forming material) as well as implementing acid treatment or heat treatment, and simultaneously molding new holes on the porous sintered body by removing the hole molding material. According to the method, the conductive polymer-containing solution passes through newly formed holes, thereby increasing the permeability.
However, the method according to Japanese Unexamined Patent Application Publication 2001 345238 involves a lot of work, as it requires a process for adding the hole molding material as well as a process for removing it. Thus, demand has been made to manufacture an anode for a solid electrolytic capacitor in which holes are formed for the conductive polymer-containing solution to pass through, without using the hole molding material. To date, a method that does not use any hole or pore forming material has not been known.
In one aspect of the present invention, the present invention takes into account these circumstances and provides a tantalum or niobium powder that can be used to manufacture an anode for a solid electrolytic capacitor in which holes (passageways or voids or pores) are formed for the conductive polymer-containing solution to pass through, without using a hole or pore molding (or forming) material. Furthermore, the present invention intends to provide an anode for a solid electrolytic capacitor that can be used in manufacturing a high-volume and low ESR solid electrolytic capacitor.
The present invention, in at least one embodiment, has also been created in consideration of said circumstances, with the intention of providing a metal powder composed of tantalum or niobium that achieves both sinterability and fluidity and the manufacturing methods thereof.
The present invention also takes into account the circumstances described above and intends to provide a method of making a metal powder that allows the manufacturing of metal powder having a given diameter range from a raw powder at high yield, and preferably without requiring a lot of work and time. The present invention further intends to provide a powder having a given diameter range.